1. Field of the Invention
The present invention relates generally to apparatus for transmitting and displaying a still picture and, more particularly, to a video telephone that can transmit and receive a still picture by utilizing a standard telephone network line.
2. Description of the Background
When a so-called video telephone is realized, a special telephone network line having a wide bandwidth is required if the video signal is to be transmitted without extensive modification. Thus, individuals typically cannot afford a video telephone, and it is relegated to corporate use where the video telephone is used for important conferences, for example.
Therefore, to be economically feasible a monochromatic, or black and white, video telephone for personal use has to employ an ordinary telephone network line. The ordinary telephone network line, however, typically is used to transmit only an audio signal and its transmission bandwidth lies in the range from about 300 to 3400 Hz. Accordingly, in this case, the image to be transmitted must be a still picture, with its video signal timebase-expanded and then transmitted.
Further, if all pixels or picture elements of one frame forming the still picture are transmitted, such transmission requires a lot of time, which is not practical in a video telephone. Therefore, the picture elements must be transmitted at predetermined intervals.
Considering the foregoing, the following standardization is proposed. Different screen modes (video formats) are described, and there are proposed modes A and B, as shown on FIGS. 1A and 1B, respectively.
FIG. 1A shows Mode A, in which there are 160 horizontal picture elements and 100 vertical picture elements, with more than 16 levels of gradation.
FIG. 1B shows Mode B, in which there are 96 horizontal picture elements and 100 vertical picture elements, also with more than 16 levels of gradation.
Assuming that there are provided 16 level gradations, then one picture element can be expressed by 4 bits. Thus, the amount of image data forming the picture screen in Mode A is expressed as: 160 picture elements.times.100 picture elements.times.4 bits=64 kbits. Similarly, the amount of image data forming the picture screen in Mode B is expressed as: 96 picture elements.times.100 picture elements.times.4 bits=38.4 kbits ,
FIG. 2 represents a signal format for use when image data is transmitted in which, when an outgoing call is to be made, a picture send key on the transmitting-side video telephone is depressed, and a dual tone signal DLTN is transmitted from the transmitting side to the receiving side for a predetermined period T.sub.1, for example, 0.4 seconds. The reception of dual tone signal DLTN at the receiving side video telephone causes it to be switched from the standard communication mode to the picture receiving mode. To this end, the dual tone signal DLTN has a constant level and is formed by mixing signals S.sub.1 and S.sub.2. Signals S.sub.1 and S.sub.2 are produced by frequency-dividing an alternating signal Sc having, for example, a frequency fc of 3.579545 MHz, which is identical to the color subcarrier frequency in the NTSC system, into frequencies f.sub.1 and f.sub.2 that are expressed as: EQU f.sub.1 =fc/1784=2006 Hz (S.sub.1) EQU f.sub.2 =fc/2192=1633 Hz (S.sub.2)
Accordingly, the signal components of frequencies f.sub.1 and f.sub.2 of the dual tone signal DLTN continue for a period T.sub.1, as shown in FIG. 2, so that the dual tone signal DLTN can be easily distinguished from an audio signal at the receiving side.
The period T.sub.1 is followed by a blank or nonsignal period T.sub.2, of, for example, 0.4 seconds. A frame synchronizing signal FSYN is transmitted during the next time period T.sub.3 having a period of time equal to about 0.1 seconds. The frame synchronizing signal FSYN serves as a standard signal for frequencies and phases of the succeeding signals, as well as a signal that becomes a standard for timing relations of the succeeding signals. The frame synchronizing signal FSNY is a frequency-divided signal S.sub.3 that is produced by frequency-dividing the signal Sc to a frequency f.sub.3 expressed as: EQU f.sub.3 =f.sub.c /2048=1748 Hz (S.sub.3)
It is to be noted that the frame synchronizing signal FSNY is formed by combining the signal S.sub.3 having the phase of 0.degree. with the signal S.sub.3 having the phase of 180.degree. in a predetermined order. The level of the frame synchronizing signal FSNY is determined to be the maximum of the signals to be transmitted. More specifically, the period T.sub.3 has a time length equal to 176 cycles (=0.1 second) of the signal S.sub.3.
An amplitude correcting signal ACAL is transmitted over the next time period T.sub.4 that has, for example, a time duration of about 0.07 seconds. The amplitude correcting signal ACAL is used to correct the level of subsequent signals caused by variations in the transmission gains in the telephone network line of the receiving side video telephone. Therefore, the amplitude correcting signal ACAL results from amplitude-modulating and phase-modulating the signal S.sub.3 by a predetermined level (16 gradations) and phase (0.degree. or 180.degree.), respectively, and is transmitted over a time period of 128 cycles. Thus, the duration of the period T.sub.4 equals 128 cycles of the signal S.sub.3.
An identification code ID is transmitted during the next period T.sub.5. The identification code ID is formed of a binary code of "0"and "1"that presents the mode of the picture to be transmitted and the communication capability of the transmitting-side video telephone, so that the identification code ID is a signal provided in such a manner that the signal S.sub.3 is phase-modulated by "0"or "1". The length of the period T.sub.5 of the identification code ID is selected to be an integer multiple of 64 cycles of the signal S.sub.3.
Image data is then transmitted during the period T.sub.6 that follows period T.sub.5. In this case, the image data is transmitted in such a fashion that the signal S.sub.3 is amplitude-and phase-modulated by image data and the resultant modulated signal Sm is transmitted.
As shown in the waveform diagram of FIG. 3, in the case of 16 gradations one cycle of the modulated signal Sm (S.sub.3) is allotted to the image data of one picture element, and the amplitude and the phase of this signal Sm are modulated in accordance with one picture element of the image data, so that when the picture element is at the black level (0th gradation) the phase of the signal Sm is 0.degree. and the amplitude thereof is maximum, whereas when the picture element is at the white level (15th gradation) the phase of the signal Sm is 180.degree. and the amplitude thereof is also at the maximum level.
The modulation of the signal Sm is limited so that even when the amplitude of the signal Sm is minimum, its modulated amplitude is not zero. Thus, when the amplitude of the signal Sm is minimum, the signal Sm is not lost, thereby providing the signal S.sub.3 as a carrier signal. If there are more than 16 gradations, then the amplitude of the signal Sm is subdivided accordingly.
Mode A forms a picture of 160 picture elements.times.100 picture elements and, thus: ##EQU1## In a similar fashion, mode B results in: ##EQU2##
The above-noted video telephone is desired to be capable of providing the two picture modes A and B, which have a different relative size picture. To realize the foregoing, it may be considered that the picture (96 picture elements.times.100 picture elements) in mode B is displayed at the central portion of the picture screen while the picture (160 picture elements.times.100 picture elements) of the mode A is fully displayed on the picture screen.